MFC 318388: Add sglist_append_sglist().

This function permits a range of one scatter/gather list to be appended to
another sglist. This can be used to construct a scatter/gather list that
reorders or duplicates ranges from one or more existing scatter/gather
lists.

Sponsored by: Chelsio Communications

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add sglist functions for working with arrays of VM pages.

sglist_count_vmpages() determines the number of segments required for
a buffer described by an array of VM pages. sglist_append_vmpages()
adds the segments described by such a buffer to an sglist. The latter
function is largely pulled from sglist_append_bio(), and
sglist_append_bio() now uses sglist_append_vmpages().

Reviewed by: kib
Sponsored by: Chelsio Communications

Reassign copyright statements on several files from Advanced
Computing Technologies LLC to Hudson River Trading LLC.

Approved by: Hudson River Trading LLC (who owns ACT LLC)
MFC after: 1 week

Add sglist_append_bio(9) to append a struct bio's data to a sglist

Reviewed by: jhb, kib (long ago)

Spelling fixes.

This patch fixes two bugs in sglist(9) and improves robustness of the API via
better semantics if a request to append an address range to an existing list
fails.
- When cloning an sglist, properly set the length in the new sglist instead of
leaving the new list empty.
- Properly compute the amount of data added to an sglist via
_sglist_append_buf(). This allows sglist_consume_uio() to properly update
uio_resid.
- When a request to append an address range to a scatter/gather list fails,
restore the sglist to the state it had at the start of the function call
instead of resetting it to an empty list.

Requested by: np (3)
Approved by: re (kib)

Change the 'resid' parameter to sglist_consume_uio() from an int to a
size_t to match the recent type change of the uio_resid member of struct
uio.

Approved by: re (kib)

Add a simple API to manage scatter/gather lists of phyiscal addresses.
Each list describes a logical memory object that is backed by one or more
physical address ranges. To minimize locking, the sglist objects
themselves are immutable once they are shared.

These objects may be used in the future to facilitate I/O requests using
physically-addressed buffers. For the immediate future I plan to use them
to implement a new type of VM object and pager.

Reviewed by: jeff, scottl
MFC after: 1 month